A LAN may include a set of common hosts with a common set of requirements that communicate over a single medium (e.g., wire). IEEE 802.3 provides standards associated with one type of LAN (Ethernet). A VLAN may also include a set of common hosts with a common set of requirements. Members of the VLAN may communicate as if they are attached to the same medium (e.g., wire) but they may not be actually attached to the same medium. Members of the VLAN may be physically located in different LANs. Therefore, a network device(s) may be required to connect the LANs in which the VLAN elements are found. A bridge is one such network device. A bridge operates at layer two (data link) of the well known OSI protocol stack (application, presentation, session, transport, network, data link, physical). The data link layer is generally concerned with physical addressing (e.g., MAC, LLC). A bridge may connect two LANs. Connecting the two LANs may include processing LAN element addresses. More generally, a bridge may connect multiple network segments at the data link layer. Since bridging takes place at the data link layer, a bridge may process information (e.g., addresses) from each frame of data received. The IEEE 802.1ad amendment to IEEE 802.1Q provides standards associated with one type of bridge (e.g., PB). 802.1Q assigns frames to VLANs by filtering. 802.1Q assumes the presence of a spanning tree and an explicit tagging scheme with one level tagging. A frame may include, for example, a destination address, a source address, a tag consisting of an ether type and a tag control information section, a MAC length/type, data, some pad, and an FCS. The tag control information section may include a priority field, a CFI, and a VID. VLAN membership may be identified by different methods including, for example, by port, by MAC address, by protocol type, by IP subnet address, and so on.
In some cases LANs may not be directly connectable by a single bridge or even by a pair of bridges. Therefore a collection of network devices may be required to connect the LANs. One collection of devices may be referred to as a PBB network (a.k.a. PBB-cloud). IEEE 802.1ah provides standards associated with one type of PBB network that can be used to connect different LANs in which elements of a VLAN may be found. Recall that a VLAN may include elements that are located in different physical LANs that are connected using PBs and a PBB network. IEEE 802.1Q provides standards associated with VLANs. For example, 802.1Q defines how a LAN frame (e.g., Ethernet frame) may be “tagged” with a header that is added to the frame. The VLAN tag may include a VID that is added to the frame as it is switched out of the LAN and onto a shared backbone (e.g., PBB). However, information (e.g., addresses, service identifiers) associated with a first LAN may be independent from information associated with a second LAN. Additionally, information (e.g., addresses, service identifiers) and/or formats (e.g., address lengths, service identifier lengths) may be independent between a LAN and a PBB. Yet information is communicated between VLAN members residing in separate LANs that are connected by a PBB. The PBs or PBBs construct an active topology of LANs (e.g., a spanning tree) for forwarding data on VLANs. Multiple VLANs can share a single spanning tree. Routing decisions for how to route a specific address over the active topology of a VLAN may be learned over time. Once again standards provide guidance on topology creation, address learning, and address un-learning.